An optical memory device has been focussed on since it is a memory device having high density and large capacity. The reason why the optical memory device has the high density and large capacity is that a bit, which is a recording unit for information, is restricted only to a light beam diameter and thus the light beam can be set to have a shape of about 1 micron.
However, this gives many restrictions to the optical memory device. More specifically, the light beam should be positioned on the optical memory device in a high accuracy in order to record information on a predetermined recording area and in order to reproduce recorded information from a predetermined recording area. In general, optical memory device, which is exclusively used for reproducing of information, can be arranged such that address information is in advance contained in a recorded bit. Therefore, it is possible to position the light beam with the reproducing of recorded information. But, it is impossible to carry out recording of information on a write once-type memory or a rewritable memory so as to contain the address information therein during the recording of information.
Accordingly, in the write once-type memory or rewritable memory, it is employed that some guiding signal and guiding address are in advance contained in a memory substrate of the write once-type memory or rewritable memory. For example, the following is a general procedure: the memory substrate of the write once-type memory or rewritable memory are provided with grooves of physical protrusions and recessions; and recording and reproducing of information are carried out along the grooves. The grooves of the physical protrusions and recessions are discrete in a circumference direction of the respective memories, thereby presenting pit information indicative of each groove's address.
When the substrate of the optical memory device is made of the polycarbonate, the injection molding process, wherein a stamper having a pattern such as guiding grooves is used, is widely employed. The following deals with a conventional method for manufacturing the stamper with reference to FIG. 3.
First, a master plate 11 having a pattern such as guiding grooves is prepared (see FIG. 3(a)). The master plate 11 is coated with an electrode member 14 such as Ni, the electrode member 14 being a thin film having a thickness the range between 20 nm and 100 nm (see FIG. 3(b)). A Ni electrocasted thin film 15, having a thickness of the range between 200 .mu.m and 400 .mu.m is formed on the electrode member 14 (see FIG. 3(c)) by the electrocasting. Then, the Ni electrocasted thin film 15 is removed from the master plate 11. Thereafter, the other side of the Ni electrocasted thin film 15 is polished (see FIG. 3(d)), thereby finishing the manufacturing of the s tamper. mentioned above, the pattern such as the guiding grooves of the master plate 11 is exactly copied to the stamper so as to reversely form a pattern such as the guiding grooves on the stamper. A good master plate 11 should be manufactured in order to obtain a good stamper.
The following two methods for manufacturing the master plate 11 have been widely known: (1) a method for forming a protrusion and recession pattern 52 on a glass substrate 51 by the use of the photo resist (see FIG. 4(a)); and (2) a method for directly forming grooves of a protrusion and recession pattern on a glass substrate 51 according to the following steps, i.e., (a) making a mask by forming a pattern by the photo resist on the glass substrate 51, (b) carrying out the dry etching to the glass substrate 51 and thereafter (c) removing the photo resist (see FIG. 4 (b)).
When the master plate manufactured according to the method of FIG. 4(a) is observed by the STM (Scanning Tunnel Microscope), it is confirmed that there exist a plurality of minute physical protrusions and recessions on the surface of the protrusion and recession pattern 52 of the photo resist. In contrast, when the master plate manufactured according to the method of FIG. 4 (b) is observed by the STM, it is confirmed that smooth pattern such as the guiding grooves is formed by removing the photo resist on which there exist the residual minute physical protrusions and recessions after the dry etching. Accordingly, the master plate made by the etching is more preferable than that by the photo resist in order to decrease the noise.
The following conventional methods (1) to (3) are well known as the method for directly forming the guiding grooves and the like on the glass substrate. The following deals with the conventional methods (1) to (3) with reference to FIGS. 5 through 7.
According to the conventional method (1), guiding groove patterns 22 are formed on a glass substrate 21 by the photo resist (see FIG. 5(a)). After the photo resist, the dry etching is carried out by the use of a gas such as CF.sub.4, CHF.sub.3 (see FIG. 5(b)). After the dry etching, the residual photo resist is removed (see FIG. 5(c)), thereby directly forming physical protrusions and recessions on the glass substrate 21.
According to the conventional method (2), guiding groove patterns 32 are formed on a glass substrate 31 by the photo resist (see FIG. 6(a)). The guiding groove patterns 32 are baked at the temperature of more than the softening point of the photo resist (see FIG. 6(b)). Thereafter, the dry etching is carried out by the use of a gas such as CF.sub.4, CHF.sub.3 (see FIG. 6(c)). After the dry etching, the residual photo resist is removed (see FIG. 6(d)), thereby directly forming physical protrusions and recessions on the glass substrate 31.
According to the conventional method (3), guiding groove patterns 42 are formed on a glass substrate 41 by the photoresist (see FIG. 7(a)). The dry etching is carried out with respect to the guiding groove patterns 42 by the use of a gas such as CF.sub.4, CHF.sub.3 with O.sub.2 gas (see FIG. 7(b)). After the dry etching, the residual photo resist is removed (see FIG. 7(c)), thereby directly forming physical protrusions and recessions on the glass substrate 41.
In order to copy the shape of the guiding grooves of the stamper to the polycarbonate substrate according to the injection molding process, it is required that the mold release of the polycarbonate substrate from the stamper is carried out while each shape of the guiding grooves of the stamper is kept unchanged, the polycarbonate substrate being formed by carrying out the injection molding with respect to the stamper.
However, according to the conventional method (1), each edge portion of the guiding grooves rises upward substantially perpendicularly. Therefore, the polycarbonate substrate is likely to be caught by the edge portion of the guiding groove when the mold release of the polycarbonate substrate from the stamper out, is carried thereby making it impossible for the polycarbonate substrate to keep the shape of the guiding groove unchanged.
The object of the conventional methods (2) and (3) is to improve the reliability, endurability and other functions of the optical disk memory substrate by keeping the respective continuity of a recording medium, a protecting layer, and other materials which are formed on the guiding grooves. Therefore, it can be achieved by the conventional methods (2) and (3) that each edge portion of the s tamper inclines so as to keep the continuity of the formed thin film. However, the conventional methods (2) and (3) do not ensure that each edge portion of the stamper greatly inclines so as to avoid being caught by the polycarbonate substrate during the mold release of the polycarbonate substrate from the stamper.
Bore specifically, as shown in FIG. 8, it is assumed that the injection molding is carried out by the use of the stamper which is obtained by the etched master plate according to the conventional methods (1) to (3), the stamper being composed of the electrode member 14 and the Ni electrocasted thin film 15. When the mold release of a polycarbonate substrate 16 from the stamper is carried out in a direction of the arrow of FIG. 8, each edge portion of the guiding grooves is caught by the polycarbonate substrate 16. Accordingly, the guiding groove pattern on the polycarbonate substrate 16 is deformed (see the reference numerals 16a and 16b of FIG. 8), thereby arising problem, i.e., the deformed grooves are not appropriate for leading a light beam which is converged onto the substrate.